Restricted by a chirping characteristic of a directly modulated laser (DML), in an optical communications system that supports high-speed (speed higher than 10 Gbps) and long-distance transmission (distance greater than 20 Km), the DML cannot be directly used as an optical transmission source. With a high-speed and large-capacity development trend for an optical network, advantages of low costs and low power consumption are increasingly attractive, and the DML has advantages of low costs and low power consumption.
In an existing technical solution, generally a filter is added after a DML of an optical communications system for performing spectrum shaping on an output signal of the DML, achieving purposes of controlling chirping and increasing a transmission distance.
In the prior art, however, this is mostly implemented by using a method of using discrete elements, where single-function optical elements are optically coupled by using free space. For example, a multi-cavity filter is added after a DML to implement conversion from a frequency-modulated signal to an amplitude-modulated signal, so as to achieve purposes of increasing a signal extinction ratio, controlling chirping, and implementing long distance transmission. In this implementation method, discrete elements are used; therefore, a size of an optical module is relatively large and integration is not convenient. Multiple discrete elements increase difficulty in coupling and packaging, and increase costs of coupling and packaging. If this solution is to be applied to a multi-channel transmitter, a functional element for wavelength division multiplexing further needs to be added, which further increases the size of the optical module.